UPPER PALEOZOIC FLORAS 327 
tion of additional as well as more complicated synclines. In these 
at first greatly restricted basins the sea began a great readvance, 
attended by the subsidence and deformation of the new basins under 
loading. ‘These changes were most active during the Pottsville, the 
lowest of the American Pennsylvanian series. ‘The new climatic and 
terrestrial conditions were most favorable to the extraordinary growth 
and differentiation of the plant life which spread across broad base- 
level plains and over the marshes and lagoons that flanked its long 
inland-reaching estuaries. 
Appalachian de position.—In the Appalachian trough the basin in 
earliest Pottsville time was confined to a very narrow estuary extending 
from the anthracite region of northeast Pennsylvania southward to 
Alabama where it became confluent with a small lobe of the sea 
extending northward toward southern Illinois and Indiana. Sub- 
sidence under loading was most rapid, with successive periods of sea 
advance to the north and west during all Pottsville time, at the close 
of which the water-level may have proximated its greatest Pennsyl- 
vanian extent; at least it exceeded the present limits of the coalfields.* 
The enormously expanded water-level of the Allegheny formation 
attended a westward migration of the axis of deposition, probably 
with the development of minor subordinate basins. Slight warping 
occurred during the period, especially to the southward, so that the 
connection with the Eastern Interior basin presumably migrated north- 
ward into the Kentucky region, foreshadowing the probable exclusion 
of the sea from the southern area of the present Appalachian coalfield 
during later Conemaugh and Monongahela time. 
Rapid Evolution. Floral characters definite, with many short-lived 
genera in early Pennsylvanian.—The most rapid evolution of the 
Pennsylvanian flora occurred during Pottsville time, though the 
generation of new forms continued at a slower rate into the Allegheny, 
which, together with the Pottsville, approximately represents the 
Westphalian or Muscovian in North America. 
The Westphalian is the period of Cheilanthites, Mariopteris, 
Diplothmema, Crossotheca, Eremopteris, Palmatopteris, Lonchop- 
teris, Megalopteris, Lesleya, Neriopteris, Bothrodendron, Ulodendron, 
Lepidophloios, and Whittleseya, besides a number of genera repre- 
t Bull. Geol. Soc. Amer., Vol. XV (1904), p. 267. 
